1. Field of the Invention
The present invention relates to a heavy duty radial tire of improved bead durability while achieving a light-weighted structure upon improving a lapel structure of carcass cords of a carcass or reinforcing cords of a reinforcing ply.
2. Background Art
In these years, there have been suggested tires having a bead structure (hereinafter referred to as bead-wind structure) as illustrated in FIG. 14 in which ply lapelled portions a of a carcass are wound around bead cores b by approximately a full round, and end portions a1 of the ply lapelled portions a that protrude from a top surface bs of the bead cores b in a radial direction are pinched between the bead cores b and bead apex rubbers c. Reference should be made, for instance, to Japanese Published Patent Application No. H11-321244, Japanese Published Patent Application No. 2000-219016.
According to such a structure, the ply lapelled portions a are interrupted around the bead cores b so that no stress at the time of deformation of the tire is applied to the end portions a1 thereof. It is accordingly possible to effectively restrict damages such as loosing of cords originating at such end portions a1. Moreover, since the ply lapelled portions a are of short length, it will also be of advantage that the weight of the tire can be decreased.
However, since the length of the end portions a1 is short in such a structure, it will be difficult to sufficiently engage the carcass ply so that it tends to be shifted in a direction of blow-by. As a result, a large shear strain is generated between the carcass ply and the bead cores b at inner end positions A of the bead cores b in the tire axial direction at the time of deformation of the tire so that cord loosing of the carcass cords is induced. Such a problem can be reduced by separating the end portions a1 from the bead cores b and by reducing strong bending-return (so-called spring-back) of the end portions a1 that are caused, for instance, in the course of forming raw tires. In this respect, such instances remarkably occur in case the temperature of the bead portion has excessively raised upon catching heat of brake pads or others to cause thermal softening of the rubber. It is known that such tendency is similarly exhibited also with a reinforcing ply that is provided for improving the rigidity of the bead portion.